It is known in the prior art to control the speed of a direct current series traction motor by a chopper apparatus including a thyristor switch device in series with the motor. Speed control of the motor is provided by varying the width of voltage pulses supplied to the motor such that the resulting average power supplied to the motor establishes the operational speed thereof. A charged capacitor has been used to provide a biased voltage across the conducting thyristor device for commutating the conduction of that thyristor device. A traction motor is operative in a motoring mode when the passenger vehicle is being propelled along a track, and in a braking mode when the vehicle is being stopped or decelerated.
As described in a published article in the Westinghouse Engineer for Mar. 1973, at pages 34 to 41, the average voltage applied to the motor armature is controlled by adjusting the ratio of chopper OFF time to the chopper ON time with the resulting average motor current determining the motor torque for moving the vehicle along the track.
It is well known to persons skilled in this art, when operating a d-c traction motor at a controlled deceleration or acceleration rate, the motor current is controlled, whereas to maintain a desired speed, the motor voltage is controlled.